Modern communication appliances require a very precise time base for signal processing of data to be transmitted. A so-called reference oscillator, which emits a signal at a stable frequency, is frequently used for this time base. A crystal oscillator is in turn used as the base for the reference oscillator, and has one specific resonant frequency with a high Q-factor. All of the clock signals which are required for operation of the communication appliance are derived from a signal from the reference oscillator. For example, the communication appliance uses the reference signal to tune the frequency of a transmission signal to a desired carrier frequency for the transmission of data. An accurate time base is also desirable for digital signal processing, for example for production of digital data at an intended data rate.
The continual trend toward larger-scale monolithic integration of individual components on a semiconductor body is leading to the development of tunable crystal oscillators, and thus to tunable reference oscillators. The tuning of the output frequency of a signal which is derived from the crystal oscillator makes it possible to subsequently correct aging effects and temperature effects. For example, this means that it is possible to vary the output frequency of a crystal oscillator for example by means of a variable capacitance, with the aid of an analog setting voltage. Another option is to provide a plurality of switchable capacitances and to use them to directly influence the output frequency from the crystal oscillator. An implementation such as this is also referred to as a digitally tunable crystal oscillator (DCXO).
Since, in general, only a very minor frequency change need be made to the resonant frequency of the crystal oscillator, it is necessary to provide a large number of capacitors with very small capacitances. This is necessary in order to sufficiently increase the resolution when changing the output frequency of the crystal oscillator. A signal at an accurate frequency can be maintained only by sufficiently good resolution. The large number of individual capacitors with small capacitances leads to a particularly large chip area, and thus to greater costs.
It would be desirable to achieve an adequate resolution accuracy even with a smaller chip area, in particular for adjustment of a reference oscillator.